It is well known in the art to propel and steer small water craft with the muscular effort of the rider utilizing paddles, oars, oscillating fins and the like. Specific examples of craft having such propulsion devices include U.S. Pat. No. 227,491 to Coulter; U.S. Pat. No. 4,172,427 to Kindred; and U.S. Pat. No. 4,389,195 to Sohaei.
Of these, both Coulter and Kindred disclose water craft powered by oscillating fins formed of flexible material. During lateral oscillation, a flexible fin bends due to water resistance. The resulting curvature change redirects the driving force produced by the fin from a sideward to a substantially backward direction. Consequently, the driving force is better directed for pushing the craft forward through the water and propulsion efficiency is increased.
While these devices are effective, greater efficiency is, however, still possible. This increased efficiency is best realized by studying and more closely imitating the actual swimming motion of a fish. The fish adjusts the curvature of its caudal fin in accordance with the fins angular position along its arc of movement as well as the fins speed of lateral oscillation. By doing this, the fish assures that the driving force produced by the fin is directed as nearly as possible straight backward in relation to its body for maximum propulsion efficiency at all times.
It should also be recognized that the caudal fin is not simply oscillated by the fish back and forth along a constant arc. Rather, the fin is moved in a substantially figure eight path. For creating such a undulatory propulsion, the fish (having the fore part of its body against the water to resist lateral motion) generates a wave by the transverse motion of the rear body segments. This wave passes down the body and results in the caudal fin taking up a substantially figure eight path during its lateral oscillation. Such motion of the fin not only exerts a push against the water from side to backward direction, but also slips away from the wake created during making up the curved part of the path. Advantageously, this reduces drag for still greater efficiency while also positioning the fin in a contracted position from which to push backward and outward against the water during the next power portion of the swimming cycle.
Additionally, it should be recognized that the front portion or primary body of the fish acts as a counterbalance to the natural lever arm of the rear fin. Water resistance along the sides of the body prevent lateral drift and twisting of the fish from side-to-side about the desired line of travel during fin oscillation. Thus, the pivot point of the rear fin is held fixed along the desired line of travel and the working power is efficiently transferred from one end of the fin to the other. No energy is lost to rocking movement and maximum propulsion efficiency is obtained.
If this natural fish action could be mechanically imitated for the greater propulsion efficiency and a watercraft designed to incorporate it, a significant step forward in the art could be made. Such a watercraft would be particularly appealing as an entry in the self-propelled watercraft field.